Aromatic polyimide films show a high temperature resistance, a low temperature resistance, good chemical properties, high electrical insulating properties, and a high mechanical strength, and are widely employed in various technical fields. For instance, aromatic polyimide films are favorably employed as materials for manufacturing electronic devices. It is known that an aromatic polyimide film composed of a biphenyltetracarboxylic acid unit and a phenylenediamine unit particularly shows a high heat resistance and a high modulus of elasticity. In view of the high heat resistance and a high modulus of elasticity, the aromatic polyimide film composed of a biphenyltetracarboxylic acid unit and a phenylenediamine unit is particularly favorably employable for manufacturing a flexible printed circuit board (FPC), a carrier tape for tape-automated-bonding (TAB), and a tape of lead-on-chip (LOC) structure.
In the industrial fields of FPC, TAB and LOC, the aromatic polyimide film is employed in the form of a composite sheet in which the polyimide film and an electroconductive film (e.g., copper foil) are united with each other. The composite sheet is processed (e.g., cut or punched) while it is transferred in plural steps. In these procedures, the composite sheet using the conventional aromatic polyimide film sometimes shows tearing. Further, the conventional composite sheet sometimes shows compression buckling when it is punched.
It is known that the tearing and compression buckling of the composite sheet are closely related to a low elongation of the polyimide film. Also known is that a known aromatic polyimide film giving a high elongation shows a low modulus elasticity. Therefore, studies have been done for providing an aromatic polyimide film showing a high elongation as well as a high modulus elasticity.
Japanese Patent Provisional Publication (unexamined) No. 61-264027 describes a process for preparing a polyimide film having a good dimensional stability by producing a polyimide film from a biphenyltetracarboxylic dianhydride and p-phenylenediamine to obtain a polyimide film and heating thus obtained polyimide film while it is placed under application of a low tension.
Japanese Patent Publication (examined) H4-6213 describes that a polyimide film having a ratio of linear expansion coefficient (longitudinal direction/lateral direction) in the specific range, a linear expansion coefficient in the longitudinal direction in the specific range shows a good dimensional stability.
Japanese Patent Publications No. 62-60416, No. 63-5421, and No. 63-5422 disclose that an improved aromatic polyimide film is produced by modifying release characteristics of an aromatic polyamide acid film which is formed by casting an aromatic polyamide solution on a temporary support.
Japanese Patent Publication No. H3-20130 describes a polyimide film derived from 3 to 4 components such as a biphenyltetracarboxylic acid component, a pyromellitic acid component, a phenylendiamine and a diaminodiphenyl ether.
Japanese Patent Provisional Publications No. H4-198229 and No. H4-339835 describes a process in which a substituted or unsubstituted nitrogen-containing heterocyclic compound is employed.
According to studies of the present inventors, although the aromatic polyimide films prepared by these known processes show improvement of thermal properties such as linear expansion coefficient and dimensional stability as well as improvement of elastic modulus in tension, such polyimide films show poor elongation and tear resistance (measured by Elmendorf tearing tester), or show lowering of heat resistance.
Japanese Patent Provisional Publications No. H5-263049, No. H2-28257, and No. H6-334110 disclose an improved TAB tape, a polyimide film having improved flexing property, and a polyimide film having improved punching property, respectively. In these publications, polyimide films are prepared from a tetracarboxylic acid component comprising pyromellitic dianhydride and benzophenonetetracarboxylic dianhydride or biphenyltetracarboxylic dianhydride and a diamine component comprising a linear diamine (e.g., phenylenediamine) and a flexible diamine (e.g., diaminodiphenyl ether). The polyimide film improved in its flexing property is prepared by utilizing an organic phosphorus compound. The polyimide film shows a tearing resistance in the range of 20 to 70 kgf/20 mm.
The polyimide films disclosed in these publication, however, are still unsatisfactory in the elongation, tearing resistance (measured by Elmendorf tearing tester) or flexing property, or are low in the tensile strength or modulus elasticity in tension. Further, the disclosed polyimide films show poor adhesiveness.
Accordingly, it is an object of the present invention to provide an aromatic polyimide film which is easily punched with no compression buckling and transferred with no tearing in apparatuses for processing and/or punching the polyimide film and/or attaching various electronic parts onto the polyimide film.
It is another object of the invention to provide an aromatic polyimide film which is favorably employed for manufacturing FPC, TAB tape, or LOC tape.